1. Field of the Invention
The present invention relates to a surface acoustical wave (SAW) filter and more particularly to a SAW filter with a modified transducer pattern having in general a maximum of one or less active overlaps per wavelength, which eliminates distortion in relatively narrow fractional bandwidth SAW filters, while enabling the filter to be formed on a standard ST cut of quartz. SAW transducers are also disclosed with six (6) or more fingers per wavelength configured with two (2) active overlaps per wavelength.
2. Description of the Prior Art
Surface acoustical wave (SAW) filters are known in the art. Such filters are useful in various communication systems, such as receivers, as well as radar systems because of their small size and operating frequency range, typically from about 10 MHz to about 10 GHz or better.
SAW filters are constructed from interdigital transducers, which are electroacoustical transducers formed on a piezoelectric material, such as quartz. A metal film is deposited on the piezoelectric material to form an electrode pattern at each end, defining an input electrode and an output electrode. Both the input and output electrode patterns are formed with overlapping finger patterns to create the desired frequency response characteristics. A time-varying voltage applied to the input electrode causes acoustical waves to travel across the surface of the piezoelectric material to the output electrode. The acoustical waves received at the output electrode cause a voltage to be generated at the output electrode by piezoelectric action. Alternatively, filters can be constructed by electrically coupling single transducer SAW impedance devices.
Various techniques are known for shaping the frequency response of such SAW filters. Such techniques involve manipulation of the electrode overlap pattern of at least one of the transducers used in the filter. Both withdrawal weighing and overlap weighing techniques are known. Such techniques are generally described in "WEIGHING INTERDIGITAL SURFACE WAVE TRANSDUCERS BY SELECTIVE WITHDRAWAL OF ELECTRODES" by Clinton S. Hartmann, Ultrasonics Symposium Proceedings, IEEE Cat. #73 CHO 807 8SU, pp. 423-426, 1973.
Overlap weighing relates to forming elongated electrodes of opposite polarities on opposing edges of a piezoelectric material. Conductive fingers, electrically connected to the opposing electrodes, are formed generally normal to the electrodes such that the overlap between opposite polarity fingers varies smoothly along the length of the transducer. The frequency response is shaped by varying the overlap pattern of the fingers between the electrodes.
In the withdrawal weighing technique, the weighing is accomplished by using overlaps that change abruptly from I to the full aperture of the transducer, i.e. only overlaps of zero and unity are used. Withdrawal weighing is normally used in applications, such as narrow bandwidth-bandpass filters and pulse compression filters.
Another important consideration of SAW filters is the coupling constant of the particular piezoelectric substrate material utilized. For optimum performance, the coupling constant of the substrate material must be appropriate for the fractional bandwidth of the device. For relatively large fractional bandwidth devices, relatively strong coupling constants are required to minimize insertion loss. Alternatively, for relatively narrow fractional bandwidth SAW filters, relatively weak coupling constants are used to minimize the distortion.
Various piezoelectric materials are known to be used as substrates in SAW filters, including lead zirconate-titanate (PZT), lithium niobate (LiNbO3), lithium tantalate (LiTaO3), bismuth germanium oxide (Bi12GeO20), quartz (SiO2), lithium tetraborate (Li2B4O7), zinc oxide and aluminum nitride. The coupling constants of such material vary. Typically, an ST cut of quartz is used for relatively narrow fractional bandwidth filters. However, for sharp cut-off filters with fractional bandwidths less than about 0.3%, significant distortion results if known electrode patterns are used.